PSA processes or units serve to purify or separate a feed gas stream. They generally comprise several adsorbers filled with adsorbent materials that are selective for at least one of the constituents of the feed stream. These adsorbers follow a pressure modulation cycle comprising a succession of phases that define the high pressure adsorption steps of the cycle, of decompression, of extraction of the most adsorbed components and recompression. Generally, the arrangement of the cycle is such that production is provided continuously without therefore having the necessity of providing a storage capacity.
An increasing number of industrial processes will from now on require gases with controlled purity, notably with a high and very high purity. Contamination of these gases with some impurities may bring about dramatic consequences for the unit or installation consuming said gas.
The great majority of PSA units possess regulating means enabling the purity of the product to be maintained at the required specification.
They may for example consist of adapting the cycle time. PSA processes treating synthesis gases H2/CO (H2 PSA) function at a given feed gas flow rate, with the feed gas charge coming for example from a natural gas steam reforming unit. Shortening the cycle time makes it possible to obtain a purer hydrogen fraction to the detriment however of the extraction yield, that is to say to the quantity of hydrogen actually produced.
Conventionally, the impurity content varies during the production phase. In the case where the gas produced consists of less adsorbable components, for example in the case of an H2 PSA, the concentration Yi of a given impurity i decreases very rapidly at the start of the production step and increases more slowly towards the end of the same step.
A typical example of these variations is given in FIG. 1 which represents the concentration Yi of an impurity in molar ppm as a function of the phase time; this extending from a few seconds for PSAs called rapid PSAs (RPSAs) to a few minutes or tens of minutes for more conventional PSAs. The high impurity content at the start of the phase time is explained by the fact that the adsorber in question has just been repressurized by means of a gas coming from an adsorber at the end of the production step. The gas produced in the very first instants therefore has the composition of the gas produced at the end of the production step (mirror effect).
In other units where repressurization is carried out differently, in particular in the case of the final repressurization of the feed gas, it will only be possible to observe impurity peaks at the end of the production step, the adsorbent material becoming saturated with impurities, the latter starting to leave with the production (breakthrough).
Beside these systematic fluctuations, peaks will generally appear from time to time, at the start or end of the step according to the type of PSA, corresponding to accidental contamination. These contaminations are due to the fact that the PSA unit does not react sufficiently quickly to operating changes.
Since the duration of the cycles of PSA units extends from a few seconds for PSAs called rapid PSAs (RPSAs) to a few minutes or tens of minutes for more conventional PSAs, regulation of the units should therefore react very rapidly to inevitable changes to the operating conditions (composition of the feed, pressure, flow rate, temperature, etc), to external conditions, (temperature, atmospheric pressure) and to various parameters such as the time taken to maneuver valves, the degree of opening, etc.
Various means exist for anticipating these accidental breakthroughs of impurities, or at least in order to limit their degree, but no system is totally reliable.
In many applications, it is not the average impurity content that is important but indeed the instantaneous maximum concentration. In point of fact, the impurity in question may, beyond a certain threshold, constitute a poison for the downstream unit, and produce undesirable reactions, condensing or solidifying.
For H2 PSAs, it is generally the case that it is imperative for the CO content to be limited to a value of the order of a few ppm at a maximum in order to prevent poisoning of very costly catalysts. For electronic applications, the majority of impurities should be maintained below 0.1 ppm. The CO content should also be limited in the hydrogen feed of fuel cells.
In the absence of any suitable device, it is advisable to adapt the functioning of the PSA so that the impurity content of systematic or accidental peaks remains below the specified maximum value for the impurity in question. For example, if the maximum permitted value for CO is 5 molar ppm, the corresponding H2 PSA will be regulated so as to obtain a systematic peak at the outlet of the order of 1 to 2 ppm, which will generally give a sufficient margin to pass accidental periodic peaks that will then reach 3 to 4 ppm.
It is self evident that such a type of operation leads to an average impurity content in the production that is much less than the permitted maximum content, here of the order of 0.6 to 0.8 molar ppm.
It is found that in relation to this average content, systematic peaks correspond to CO contents 2 to 3 times higher, while periodic peaks may reach 6 to 7 times this average value. The duration of these peaks for a standard H2 PSA will generally be of the order of a few seconds. More generally, they only represent in duration a small percentage of the phase time of a PSA, of the order of a few per cent.
This average purity excess, made necessary by the presence of periodic or systematic peaks, will obviously result in a much lower hydrogen extraction yield and therefore production losses or conversely in a feed gas requirement greater than the actual requirement.
For other types of units, this will inevitably result in additional energy consumption and in practice always in overinvestment.
Several systems have been proposed for improving the operation of units that should ensure production of a fluid of which one of the constituents is an impurity that must imperatively remain below the predetermined threshold.
Document FR-A-2,735,381 proposes the insertion, between the gas production unit and the user, of a reservoir containing an adsorbent material making it possible to retard the progression of an accidental peak so as to give time to analyze production, to detect the problem and to bring a halt to production that is outside specification. Such a process makes it possible to have less of a margin on the operation of the unit but therefore only partially solves the problem. In point of fact, although it makes it possible to prevent contamination of the downstream circuit in CO, this system leads to cessation of the hydrogen supply coming from the unit as soon as a peak exceeds the permitted maximum concentration.
Document U.S. Pat. No. 3,897,226 proposes to add a complementary adsorber between the gas production unit and the user, and to purify gas coming from the main unit with this adsorber as soon as the concentration of impurities exceeds a given threshold. Here also, this device makes it possible to manage accidental peaks or those being produced at sufficiently long time intervals. The system is complex in the sense that it is necessary to control the streams coming toward the adsorber and bypassing the adsorber so as to remain below the impurity threshold limit.
Document GB-A-2,113,567 proposes to add an adsorber in series with the main adsorption cycle, purifying production gas coming from the main unit in a complementary manner as soon as the latter is no longer at the required specification. This complementary adsorber is regenerated at each cycle jointly with one of the adsorbers of the main unit. This system makes it possible to limit systematic impurity peaks, but at the price of considerable complication of the main unit. This system makes it possible to respond with difficulty to the most usual case of systematic peaks at the start and end of the production step.
A certain number of PSAs have several adsorbers in production simultaneously. This is the case for example for H2 PSAs that process large gas flows for which 2, 3, 4 or even more adsorbers provide hydrogen simultaneously. Since each of the adsorbers has commenced its production cycle at a different moment, production is in some way automatically averaged out. Such a system effectively reduces the relative size of systematic or accidental peaks but does not eliminate this effect.
Finally, EP-A-748 765 describes the installation of a reservoir containing an adsorbent material having an affinity for carbon monoxide, between a unit for purifying carbon monoxide by adsorption for stopping water and CO2 and a cryogenic separating unit. The object of this unit is to regulate the carbon monoxide stream which varies cyclically: the adsorbent material provides CO when the production flow rate becomes reduced and stores CO in the opposite case. The reservoir acts as a storage capacity. In this case there is a variation of the order of a % about the average characteristics. On the contrary, the CO content in this method is not limited in pure hydrogen to the ppm level. It is no longer envisaged that fluctuations of CO content will be dealt with that may reach, or even exceed, 6 to 7 times the average content in production. It is said that the adsorbent could be a molecular sieve, in particular the same adsorbent as that used in the main purification.
As regards the stopping of CO in H2 PSA units, U.S. Pat. No. 6,605,136 teaches that in order to stop CO in hydrogen, it is possible to select one or more zeolites with or without a binder belonging to those chosen from CaA, NaX, CaX, BaX, LiX, NaLSX, CaLSX, BaLSX and LiLSX.
It is also disclosed that, according to the supply period, particles of different dimensions should be used.
It is thus recommended for example that particles are used of 0.3 to 0.8 mm diameter for adsorption periods of 0.25 to 30 seconds and of 1.2 to 1.6 mm for adsorption periods of 7 to 120 seconds.
In addition, US-A-2006/0254425 indicates that the main part of CO may be preferably adsorbed in a layer of high density active carbon, but that exchanged zeolites of the A, X, Y type, chabazite, mordenite, etc may be used. It also teaches that the residual CO content will advantageously be stopped by zeolites of the CaA, LiX, CaX, LiLSX, CaLSX, Li-mordenite, Ca-mordenite type etc. It is stated that adsorbents having a Henry constant for CO greater than 2.94 (mmol/g)/bar are preferred. It is also stated that a substance having a Henry constant greater than 10 (mmol/g)/bar adsorbs the compound to be stopped too strongly to be used in a PSA.
Finally, document EP-A-1 716 906 that claims, for an H2 PSA unit intended to stop CO, an adsorbent is to be used having a Henry constant for CO between 2.5 and 5.5 (mmol/g)/bar, a unit approximately equivalent to the unit previously used (mmol/g)/bar. From table 4, it will be seen that use will preferably be made of a 5A zeolite exchanged with between 70 and 95% calcium, of 5A without binder exchanged with between 60 and 91% and of BaX. The use of X zeolites and of 5A that is exchanged to too great an extent or is not exchanged enough with calcium will be avoided.
From this, it may be summarized that, in order to stop CO traces in a PSA, a large number of adsorbents may be used, provided that they adsorb CO sufficiently but not too much, that is to say an adsorbent having a Henry constant preferably within the range of 3 to 5 (mmol/g)/bar. In addition, the more rapid the cycle, the more small-size adsorbents will be used in order to increase the kinetics.
However, the state of the art does not disclose a simple method for limiting, at the same time, systematic impurity peaks and accidental impurity peaks of CO in the form of traces in hydrogen, knowing that these may conventionally reach concentrations that are a minimum of 6 to 7 times higher than the average CO concentration. In the hypothesis where a storage capacity would be installed in series with the PSA, the state of the art no longer states what adsorbent would be more effective, in particular if the adsorbents already used in the PSA are particularly suited or not to this role of limiting CO peaks.
Starting from this, a problem that is presented is to provide a simple application process employing an adsorbent material making it possible to ensure hydrogen production with a CO content that must be below a given value.
One solution of the invention is a purification or separation process for a stream of feed gas containing hydrogen H2 and a molar content of carbon monoxide (CO) greater than or equal to 1000 ppm, wherein:    a) said feed gas stream is introduced continuously into a first purification unit (10) of the PSA type containing a first adsorbent on which CO is preferably adsorbed;    b) the gas coming from step a) is recovered, at least partially purified of CO and having an average CO molar content T less than or equal to Tacc, it being itself less than or equal to 100 ppm and from time to time a first maximum CO content TM0 greater than Tacc;    c) all the gas coming from step b) is introduced continuously into a downstream adsorber (20) containing at least one second adsorbent (21) on which CO is preferably adsorbed;    d) the gas enriched in H2 coming from step c) is recovered having an average CO content T and a maximum CO content TM1, both being less than or equal to Tacc.
Tacc is the acceptable CO content for the downstream process. Tacc is preferably less than 50 ppm, even more preferably less than 25 ppm and even more preferably less than 10 ppm.
According to the case, the process according to the invention may have one of the following features:                in step d) the maximum CO content TM1 is such that (TM1−T)<½ (TM0−T), preferably (TM1−T)<⅕ (TM0−T), still more preferably (TM1−T)< 1/10 (TM0−T);        in step b) the maximum CO content TM0 is such that TM0≧1.1×T, preferably TM0≧2×T;        in step c) the second adsorbent has, at 20° C., a Henry constant for CO greater than or equal to 2.5 (mmol/g)/bar, preferably greater than or equal to 5 (mmol/g)/bar;        the first and second adsorbents are either identical or different when the second adsorbent has adsorption kinetics less than that of the first adsorbent;        the second adsorbent is of the granular type, in particular in the form of beads or rods, while the first adsorbent is of the monolithic type, in particular in the form of sheets;        the second adsorbent is chosen from an X zeolite, preferably an LSX zeolite, a 5A zeolite or an exchanged zeolite, in particular a zeolite exchanged to the extent of more than 50%;        the downstream adsorber contains at least two different adsorbent materials;        the feed gas stream is obtained by steam reforming, by partial oxidation of hydrocarbons or alcohols, by gasification of coal or residues, or by mixed processes;        gas coming from step d) is intended for a network of the pipeline type, a chemical unit, a petrochemical unit, a unit forming part of a refinery or for feeding a fuel cell.        
The invention will be described in greater detail by means of FIGS. 2 to 5.
In FIG. 2, the unit 10 represents a feed gas purification unit 1. This unit is for example a hydrogen PSA processing a charge flow rate of 165,000 Nm3/h of a gas coming from steam reforming, at a pressure of 20 bar abs and a temperature of 35° C. and having a composition corresponding to 70 mol % H2, 22 mol % CO2, 4 mol % CH4 and 4 mol % CO (dry gas).